Cardiac pacing involves the electrical stimulation of the heart in order to control the timing of the contractions of the heart. Electrical stimuli in the form of pulses are generated by a battery-powered pacemaker and applied to the tissue of the heart using a pacing lead. The pacing lead may additionally be used to sense intrinsic electrical activity within the heart. The sensed electrical activity may be used by the pacemaker to control the timing of pulses that are applied to the heart, or may be used to select a particular pattern or method for pacing the heart.
Since the response of the heart to a particular pacing stimulus will typically vary from individual to individual, and may further change for a given individual with changing conditions, it is desirable to have an internally implantable device which measures the performance of the heart and supplies the measurement to the pacemaker. The measurements of cardiac performance can then be used by the pacemaker to adaptively adjust the pacing stimulus. Measurements of cardiac performance can also be communicated to a physician using telemetry techniques, and the physician can use the measurements to detect various abnormalities.
One method of measuring cardiac performance involves using an internally implanted flow sensor to measure the velocity of blood flow through a blood vessel. The sensed blood flow velocity is communicated to the pacemaker over wires that connect the flow sensor to the pacemaker. The sensed blood flood velocity may be used to estimate cardiac output, which is the volume of blood pumped by the heart per unit time. The sensed blood flow velocity may further be used to detect abnormal blood flow patterns, which may be caused, for example, by an improperly functioning heart valve.
Flow sensors can also be used with implantable defibrillators to detect or verify the absence of a heart beat. The defibrillator may then apply a shock to the heart to restart the heart.
As is known in the art, a thermocouple in conjunction with a heat source may be used to measure blood flow velocity. A thermocouple is a temperature sensor comprised of two dissimilar metals that are joined together to form a junction. A thermocouple may be formed, for example, by joining two wires of different metals together at one end. The junction between the wires may be placed at a measurement point while the opposite (i.e., unconnected) ends of the wires are held at a reference temperature. A voltage produced by the thermocouple across the unconnected ends of the of wires will then indicate the difference between the reference temperature and the temperature at the measurement point, and may thus be used to determine the temperature at the measurement point.
U.S. Pat. No. 5,174,299 to Nelson ("the Nelson Patent") discloses a flow sensor that uses a thermocouple to measure the velocity of blood flow. The flow sensor is adapted for placement along a pacing lead. The flow sensor comprises three metal tubes that are joined together from end to end to form two junctions, with the center tube composed of a different metal than the outer tubes. Wires connected to the junctions convey a voltage to the pacemaker that indicates a temperature difference between the two junctions. A heater provided proximate to the "upstream" junction is used to heat the upstream junction. A temperature difference between the two junctions is then monitored by the pacemaker to determine the rate at which heat is carried away from the upstream junction by the flow of blood.
The thermocouple-based flow sensor disclosed in the Nelson patent has a number of limitations. For example, the flow sensor does not detect the direction in which heat is carried away from the upstream junction, and thus effectively treats negative flow (i.e., flow in the reverse direction) and cross flow (i.e., side-to-side flow relative to the flow sensor) as positive flow. The treatment of negative flow as positive flow becomes a problem, for example, when a patient has a valve disorder that causes regurgitation (i.e., reverse flow) of blood through the valve as the heart muscle contracts. Since the flow sensor detects this reverse flow as positive flow, the pacemaker fails to detect the valve disorder. Moreover, the pacemaker counts the reverse flow as positive flow in measuring cardiac output, producing an inaccurate performance measurement. The treatment of cross flow as positive flow becomes a problem, for example, when cross flows occur in the region of the flow sensor as the result of side-to-side movement of the pacing lead. Such movement of the lead can occur as the result of the contractions of the heart. Failure to exclude cross flows from the flow measurement decreases the accuracy of the measurement of cardiac output.
The flow sensor disclosed in the Nelson patent further does not produce a zero-based voltage output (i.e., an output voltage of zero when the flow velocity is zero). This complicates accurate detection of very low and zero flow velocities. The flow sensor is thus not well suited for defibrillator applications, wherein detection of low flow rates is critical.
The flow sensor disclosed in the Nelson patent further requires the passage of two separate pairs of conductors through the lead between the flow sensor and the pacemaker or defibrillator. The first conductor pair carries the voltage generated by the thermocouple to the pacemaker. The second conductor pair carries a heating current from the pacemaker to the heater. The need for two separate conductor pairs affects the complexity of the lead, and affects the number of connectors that must be provided on the housing of the pacemaker. A need thus exists in the art to provide a thermocouple-based flow sensor that discriminates between forward and reverse flow, that is insensitive to cross flows, that accurately detects very low and zero flow velocities, and that requires the passage of only one pair of conductors between the flow sensor and the pacemaker or defibrillator.